Hydraulic fracturing is used to improve well productivity by hydraulically injecting fluid under pressure into a selected zone of a reservoir. The pressure causes the formation and/or enlargement of fractures in this zone. Proppant is typically positioned in the fractures with the injected fluids before pumping is halted to prevent total closure. The proppant thus holds the fractures open, creating a permeable and porous path, open to fluid flow from the reservoir formation to the wellbore. Recoverable fluids, such as, oil, gas or water are then pumped or flowed to the surface.
The information on the geometry of the generated hydraulic fracture networks in a given reservoir formation is critical in determining the design parameters of future fracture treatments (such as types and amounts of proppant or fluids to use), further well treatments to be employed, for the design of the future wells to be drilled, for managing production, etc. Therefore, there is a need for accurate mapping of the fractures. The methods typically used include pressure and temperature analysis, seismic sensor (e.g., tilt-meter) observational analysis, and micro-seismic monitoring of fracture formation during fracturing processes. Each of these methods have their drawbacks, including complicated de-convolution of acquired data, reliance on assumed parameters, educated “guesswork” as to the connectivity of various mapped seismic events, and problems associated with reliance on mapping-while-fracturing methods, namely, measuring the shape of the fractures during formation (rather than after closure or during production), measuring fractures which may not be conductive to the wellbore, acoustic “noise” from the fracturing procedures, and an inability to distinguish between seismic events that are caused by fracture formation or other processes.
Methods have been suggested for mapping fractures using explosive, implosive or rapidly combustible particulate material added to the fracturing fluid and pumped into the fracture during the stimulation treatment, namely, in U.S. Pat. No. 7,134,492 to Willberg, et al., which is incorporated herein by reference for all purposes. Similar methods are disclosed in Autonomous Microexplosives Subsurface Tracing System Final Report, Sandia Report (SAND2004-1415), Warpinski, N. R., Engler, B. P., et al., (2004), incorporated herein by reference for all purposes. However, the suggested practices have significant drawbacks, including the transport and handling of explosive particles at the surface and during pumping, exposure of explosive particles to very high pressures, treatment and wellbore fluids and chemistry, difficulty in controlling the timing of the explosions given their lengthy exposure to fracturing fluids, exposure of particles to significant and high pressures during fracturing, the risk of explosive particles becoming stuck in the well completion string, pumping and mixing equipment, etc. Further, some of the proposals require the inclusion of power sources, electronics, etc., in the injected particles which may be impractical at the sizes required to infiltrate a fracture and proppant and are relatively expensive.
It is therefore an object of the present invention to provide a new approach to evaluating hydraulic fracture geometry.